Anti-capillary fold

An anti-capillary fold is a small backward fold (typically 10 to 15 mm) on the uphill edge of a metal flashing, wall cladding sheet, or roof sheet. The fold creates a tiny upward-facing channel that breaks the capillary path water would otherwise wick up the join. Without an anti-cap fold, capillary action can draw water against gravity up to 30 mm or more, allowing water to enter the cavity even at well-overlapped joints.

Why capillary action matters. Water rises in narrow gaps (less than ~3 mm wide) because surface tension overcomes gravity. On a flashing lap or sheet overlap, the lap line is exactly the kind of narrow gap that pulls water uphill. Once water is uphill of the lap end, it drips into the cavity and onto the substrate, regardless of how steep the roof or how long the overlap.

Regulatory basis. ABCB Housing Provisions 2022 Part 7.2 (Roof drainage and roof flashings) requires anti-capillary folds on flashing laps with a minimum 75 mm overlap in the direction of fall. AS 1562.1 (Design and installation of sheet roof and wall cladding) reinforces this for metal roofing.

Where anti-cap folds are required:

Apron flashings (where a roof meets a vertical wall above).
Step flashings (at intersections of roof and wall on a sloping roof).
Box-gutter and saddle flashings at the junction with the wall.
Cap flashings over parapets and chimneys.
End-lap joins in low-pitch roofs.
Penetration flashings for vent pipes, flues, and skylights.

The fold profile. A typical anti-cap fold:

Depth: 10 to 15 mm.
Angle: folded back about 30 to 60 degrees from the flashing plane.
Orientation: the fold edge points uphill so water hits the fold underside, not the top.
Continuity: runs continuously along the whole lap edge; no breaks for fixings.

Common defects:

Fold missing or trimmed off by the installer to make the flashing sit flush. Water tracks past the lap within months.
Fold pointing the wrong way (downhill) so the capillary path is preserved.
Fold flattened during install by being knocked or stood on, breaking the capillary break.
Fold not continuous because the fitter trimmed it at a corner. Water finds the trimmed section.

For builders. Two practical points:

Spec pre-formed flashings with anti-cap folds. Sheet metal suppliers fabricate to order with the fold profile built in; field-folding on a brake produces inconsistent results.
Inspect at the fold line, not just the lap. Visual inspection of a finished flashing rarely reveals a missing fold. Run a hose-test before the roof goes on if the flashing is critical to weatherproofing.

Also known as: anti-cap fold, kicker (informal), capillary break.

Category: Practical / metal roofing / weatherproofing.

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